A porous-shell particle typically comprises a metal oxide core particle surrounded by a porous shell around the core particle. Porous-shell particles are typically either “totally porous,” having a porous core and a porous shell, or “superficially porous,” having a substantially solid core and a porous shell. Porous-shell particles are used in a variety of applications, including for example, catalysis and chromatography. For most applications, micron scale porous particles are used, typically having diameters less than 500 μm.
A number of methods exist for making porous-shell particles. One method involves a spray-drying technology, which is described in U.S. Pat. No. 4,477,492 to Kirkland. In the spray-drying method, silica cores are mixed with colloidal silica sol, and the resultant mixture is spray-dried under high pressure at an elevated temperature (typically around 200° C.). While this method has its advantages, particles made by spray-drying are often incompletely or inhomogeneously coated and are often contaminated by undesired particles formed without the silica core, which can be difficult to separate from the desired particles.
Another method for making porous-shell particles involves multilayer technology, wherein metal oxide core particles are repeatedly coated with alternating layers of colloidal particles through electrostatic deposition. Methods using this approach are described in U.S. Pat. No. 3,505,785 and U.S. Patent Application No. 2007/0189944, both to Kirkland. The multilayer layer method, however, can be time-consuming, often requiring multiple deposition steps. In addition, the multilayer method is typically not optimal for particles having diameters less than 5 μm.
Another approach is the coacervation method, wherein metal oxide core particles are coated with a coacervation layer comprised of an organic material (typically a polymer) and colloidal metal oxide particles. The organic material is then removed, leaving behind metal oxide core particles having a porous shell comprising the colloidal particles. The coacervation method is more amenable to large-scale production relative to other methods, but is still not optimal. Typically, difficulties arise in forming the coacervation layer. In many instances, the coacervation layer does not properly coat the metal oxide core, which undesirably results in the formation of totally porous particles comprising the colloidal metal oxide particles together with bare metal oxide core particles.
Accordingly, there is a need for improved methods for making porous-shell particles, and in particular methods which can provide improved particle and pore size distribution, as well as smaller porous-shell particles. These needs and other needs are satisfied by the present invention.